The daughter of Morad Tahbaz, a British American citizen detained in Iran, said she has a “fear of what happens next”, following her father's release from Evin Prison in Tehran.
Tara Tahbaz described how she felt “ecstatic” and overcome by a “sense of relief” when she spoke to her father in their first video call after the breakthrough.
However, she said the happy feelings were tinged with a fear for the future, while her family remains “optimistically hopeful”.
Washington last week agreed a deal with Iran that, if everything goes to plan, will see Mr Tahbaz and four other American Iranians returned to the US.
Mr Tahbaz was among four Iranian American prisoners released from the capital's notorious Evin Prison and placed under house arrest under the terms of the agreement.
“Being able to see that first video call, I just was ecstatic and a [had] a sense of relief but also fear of what happens next,” she said. “We don't know what happens next,” Ms Tahbaz said in interview with MSNBC.
She said the US deal with Iran to secure the release of five hostages, is the most progress since her father's ordeal began in 2018.
But she and her relatives will not celebrate until Mr Tahbaz, 67, arrives back on American soil, she said.
The businessman and wildlife conservationist holds American, British and Iranian citizenship.
He was arrested in his country of birth in January 2018 during a crackdown on environmental activists.
Tehran accused him and seven other conservationists of collecting classified material about the country’s strategic areas, under the guise of environmental work.
His Canadian Iranian colleague, Kavous Seyed-Emami, died in custody a few weeks later in unexplained circumstances.
Mr Tahbaz was charged with “corruption on Earth”, which carries the death penalty. His charge that was later changed to “co-operating with the hostile state of the US”.
In March 2022, Mr Tahbaz was sidelined in a deal between the UK government and Tehran that saw fellow detainees Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori returned to Britain.
Under the £400 million agreement, the then-foreign secretary Liz Truss said she thought that Mr Tahbaz would be freed from jail on furlough.
However, after only two days he was returned to his cell. The Iranians claimed there had been a misunderstanding within his family.
“It’s really overwhelming,” Ms Tahbaz said after the deal to secure the release of her father was announced.
“I think that there’s a big sense of relief knowing that he’s no longer in the prison but as I mentioned … they’re not free until they’re home with us.
“So, we’ll be anxiously waiting until we actually get to embrace them and hold them. But [we are] so grateful that this step was taken. It’s the farthest that we’ve ever got to seeing hopefully the light at the end of the tunnel to have them back with us.”
Her sister Roxanne Tahbaz previously accused the UK government of ignoring her pleas for assistance.
Neda Shargi, whose brother Emad Shargi is included in the US-Iran deal, also spoke to MSNBC.
Ms Shargi expressed her gratitude to the US government for making progress on the complex cases.
She said she intends to continue speaking out until the men are back home in the US to remind people that “these are American citizens that have been taken because they’re American”.
“They need to come home for their families, and to their country,” she added.
The White House last week confirmed the release of the Americans and said it “will not rest until they are all back home in the United States”.
Businessmen Siamak Namazi and another American citizen, whose identity has not been made public, are also included in the deal.